A known technique for distributing pulses of seismic energy to the earth's strata in marine seismic exploration is to use an array of chambered airguns attached to the aft of a boat. Such arrays typically consist of a number of sub-arrays having individual airguns and clusters of airguns rigidly mounted in frames. The boat tows this array to a predetermined position and then fires the chambered airguns at the same time. This technique is more fully described in U.S. Pat. No., 4,956,822, which is incorporated herein by reference.
Each of the chambered airguns in the array is operable to generate a series of pulses of seismic energy that travel through the water and into the earth's strata. Each airgun produces a "signature" which provides a quantitative measurement of the airgun's output and performance. When an airgun is fired, an initial pulse is generated having a first, positive peak called the "direct" or "primary" arrival. The amplitude of this peak is called the peak amplitude, which corresponds to the increase in pressure that occurs in the water as the compressed air first explodes out of the airgun. The primary arrival is followed by a large negative peak that corresponds to a reflection from the waters surface and is called a "ghost".
The primary arrival and ghost are followed by a sequence of bubble pulses or oscillations which correspond to the pressure variations that occur in the water due to the expansion/compression cycles of the air bubbles.
One of the characteristics of the airgun's signature is the primary pulse-to-bubble ratio. This is a ratio of the primary peak-to-peak amplitude, divided by the peak-to-peak strength of the bubble pulses. This pulse-to-bubble ratio varies from airgun to airgun, and is used to characterize the various types of airguns. This ratio varies as a function of chamber volume, airgun depth and firing pressure. Another characteristic of an airgun signature is the bubble period. This is the time delay between the primary peak amplitude and the first bubble peak amplitude. The bubble period for an airgun depends on the chamber volume, airgun depth and firing pressure.
As previously mentioned, an airgun array typically includes a plurality of airguns either individually, as clusters, or a combination thereof. The primary purpose of such an arrangement is to increase the total output energy, and also allows the designer of the array to shape the far-field signature and the directivity pattern, or "tune" the array, such that the primary peaks add up or constructively interfere with one another, while the bubble oscillations cancel themselves out or destructively interfere with one another.
Generally speaking, airgun arrays are designed and used with the assumption that each element of the array will remain at a fixed vertical position relative to a fixed reference during a marine seismic operation, which is normally a valid assumption during calm sea conditions. However, when the array encounters rough sea conditions, the large waves can have a pronounced effect on the vertical position of each element of the array. In such a situation, the primary pulses from each element of the array are no longer synchronized, and the bubble pulses are not effectively suppressed. This, of course, defeats the primary purpose of constructing an array out of a plurality of individual elements. In very rough sea conditions, the array can become "de-tuned" to the point where the marine seismic operation has to be halted, which leads to down time and increased costs.